Continued development of physically larger and higher power integrated circuits is focusing much attention on the need for maintaining the operating temperature of the circuits at a point where the reliability of the circuit is at an acceptable level. The higher operating temperatures of the electronics has increased attention on the thermal expansion mismatch between the heat sink the circuit wiring board and the heat dissipating components mounted proximate the circuit wiring board. The large mismatch in thermal expansion coefficients between electronic components, circuit boards and heatsinks is responsible for fatigue stress, and subsequent fatigue failure of solder joints, a common cause of failure in surface mount assemblies. Due to these stress and fatigue problems, the use of surface mount technology has not proceeded nearly as rapidly as anticipated.
Previous attempts at solving these problems have been only partially successful. The prior art solutions include laminating a circuit board to a heatsink comprising a copper/Invar/copper laminate or copper/molybdenum/copper laminate. The thickness of the various materials, their Young's moduli, and their individual thermal expansion coefficients determine the coefficient of thermal expansion of the circuit board/heatsink combination.
Unfortunately, this approach suffers from many problems. First, the copper/Invar/copper laminate and the copper/molybdenum/copper laminate are quite heavy. In fact, these materials weigh almost three times as much as aluminum. In many aerospace applications, excess weight translates into thousands of dollars in added costs.
Second, these heatsinks have thermal coefficients of expansion greater than that of silicon. Attempts at lowering the thermal coefficient of expansion of the copper/Invar/copper sandwich by adding a greater percentage of Invar to the laminate results in the entire composite becoming a relatively poor heatsink since Invar is not a good thermal conductor.